Trouble is, detectors haven’t evolved much in recent decades. So, Fadell went to work on designing one that takes advantage of today’s technologies. (“We think there’s a better way,” Fadell says.)

The result? Nest Protect, a square-shaped device with a perforated sunflower pattern that looks little like smoke detectors of decades past and more like a small set-top box or Mac Mini computer. Expected to arrive next month at U.S. retailers including Amazon (AMZN), Apple (AAPL), Best Buy (BBY), and Home Depot (HD), Nest Protect will sell for $129 in white wired and battery-powered models. A black version will be sold exclusively via Nest’s website.

The Nest Protect changes the way users are notified of emergencies. With a feature called “Heads Up,” a human voice pipes up with a message like, “Heads up. There’s smoke in the dining room.” It serves as an early warning if the device detects slowly rising smoke or CO levels. Instead of a traditional alarm, a female voice may also sound off at night in a home where children are sleeping, a decision Fadell made after reading a study that suggested that children are more likely to respond to a person’s voice as opposed to loud beeping tones. If the threat is immediate, Nest Protect bypasses Heads Up and sounds an alarm instead. Regardless, turning off the alarm is a simple matter, thanks to built-in ultrasonic and activity sensors. Instead of climbing on top of a chair, one just has to wave their hand within six to eight feet of the device.

A ring of LED lights serves several functions. It lights red during an emergency and briefly goes green after a user turns the lights off in the evening, to indicate sensors and battery energy levels are in good shape. The LED lights also illuminate when someone walks under them at night or merely wants a night light. And because Nest Protect has Wi-Fi connectivity, one model can communicate with others in different rooms, notifying someone in the bedroom about a basement fire. Wi-Fi also enables the user to control Nest Protect with an updated version of the same app used for Nest Thermostat.

Releasing a smoke detector as its second product was the next logical choice for Nest, which employs around 170 people and has raised at least $80 million from backers including Kleiner Perkins, Google Ventures (GOOG), and Shasta Ventures. Fadell had kicked the idea around for a while, and some Nest Thermostat owners had asked for one. There’s also simply more business in the household alarm product category. “Three to four times more smoke detectors are sold per year than thermostats,” Fadell admits.

Nest will not disclose how many thermostats it’s sold since 2011, but customer satisfaction is high: 96% of users are likely to recommend a Nest product to a friend, for instance. It has also been good for the environment. According to Fadell, over 1 billion kilowatt hours in energy have been saved thanks to Nest’s power-efficient hardware and software. (That’s more than 15 minutes of America’s overall annual energy consumption.)

The Nest CEO remains mum on future plans beyond bringing Nest Thermostat to the U.K., a territory he thinks could benefit. Says Fadell: “Your TV has changed dramatically. Your phone has changed. There are other products in the house that haven’t. So we want to go after it.”